An electric storage element used in electrochemical devices such as an electric double layer capacitor, lithium ion capacitor, redox capacitor, and lithium ion battery has laminated structure wherein active material layers are placed between an ion permeable layer and a first collector layer and between the ion permeable layer and a second collector layer. Also, such electrochemical devices encapsulate the electric storage element in a package.
The active material layers in such an electric storage element are generally formed by applying an active material paste, containing an active material powder, a binder, and a conductive assistant, in a layer and then by drying the applied active material paste such that the active material layers are disposed between the ion permeable layer and the collector layers (the first and second collector layers). The outer edges of the active material layers are exposed externally from the electric storage element, like those of the ion permeable layer and the collector layers.
Therefore, there is a possibility that a bit of the active material powder may fall off the outer edges of the active material layers, due to an external force such as vibration or impact exerted while transporting the electric storage element or an electrochemical device containing the electric storage element, or due to an internal stress created during operation of the electrochemical device (e.g., an internal stress created by a gas produced at charge or discharge).
Since the active material powder is a conductive material, the fallen bit of the active material powder may cause functional disorders such as a short circuit between positive and negative electrodes, lowered charge or discharge capacity, or increased leakage current.
To prevent such functional disorders, some kinds of methods have been under development. One example of such attempts is to increase the proportion of the binder in the active material so as to enhance the bind in the active material powder. However; such an increased proportion of the non-conductive binder may increase the resistivity of the active material layer and lower the performance of the same.